Optical telecommunications generally involves the use of light beams propagating through optical fibers to carry data. As the light beams travel through the fiber, they may be distorted by the fiber in a number of ways. One type of distortion caused by optical fiber is polarization mode dispersion, or "PMD".
PMD refers to an effect that an optical device, such as a span of optical fiber, has on the separate polarizations of a light beam. A light beam can be approximated as having electrical components that vibrate at right angles to the direction of travel. In the simple case of a short fiber section, the polarization or state of polarization of the light beam can be thought of as the direction of these right angle vibrations, where the light beam travels in a straight line. In the more general case, these components are superimposed in a more complex way.
The most convenient way to fully represent PMD and its effects is as a three dimensional vector. Generally the PMD vector is represented using Stokes space, a three-dimensional geometrical space, and the Poincare sphere, a sphere within Stokes space where every possible polarization state maps to a specific (and different) point on the sphere. Three axes, S1, S2, and S3, define this three dimensional space and any polarization can be described in reference to these axes. Thus, any polarization can be described by its S1, S2, and S3 components. The S1, S2, and S3 components of a polarization are called its Stokes components.
One effect of PMD is to cause the polarization of a light beam at the output of a fiber section, i.e., the output polarization, to vary with frequency when the polarization of the light beam at the input of the fiber section, i.e., the input polarization, remains fixed. In other words, because of PMD, a light beam having a given input polarization will have a first output polarization when the light beam is injected into the fiber at a first frequency .omega..sub.1 and a second output polarization when the light beam is injected into the fiber at a second frequency .omega..sub.2, where the second frequency .omega..sub.2 differs from the first frequency .omega..sub.1 by some frequency interval .DELTA..omega., i.e., .omega..sub.2 =.omega..sub.1 +.DELTA..omega..
In addition, PMD itself varies with frequency. Thus, referring to the previous example, the PMD calculated using the measurements taken at the frequency pair of .omega..sub.1 and .omega..sub.2 would be different from the PMD calculated using measurements taken at another frequency pair such as .omega..sub.2 and .omega..sub.3, where .omega..sub.3 =.omega..sub.2 +.DELTA..omega., or .omega..sub.3 and .omega..sub.4, where .omega..sub.4 =.omega..sub.3 +.DELTA..omega.. Consequently, the PMD for a given optical device, such as a section of fiber, is measured over a frequency range.
An example of a current method for measuring PMD is the Poincare Sphere Technique, or "PST". For each PMD determination, two different input polarizations are injected into an optical device under test, such as a fiber section, at each frequency of a frequency pair and the output polarizations are measured. Specifically, a light beam having a first input polarization is injected at the first frequency of the frequency pair into an optical device under test and the output polarization measured. Then, a light beam having this same first input polarization is injected at the second frequency of the frequency pair into the device under test and a second output polarization is measured. Third, a light beam having a second input polarization is injected at the same first frequency of the frequency pair into the device under test and a third output polarization is measured. Finally, a light beam having this same second input polarization is injected at the same second frequency of the frequency pair into the device under test and a fourth output polarization is measured. Depending on the results, a different first polarization may have to be chosen and the process repeated. The PMD for this first frequency pair is then calculated. This same procedure is used to determine the PMD for the other frequency pairs remaining in the frequency range being tested.
FIG. 1 shows a block diagram of a general apparatus capable of carrying out the previously described method. Control block 50, which could be a computer, directs tunable laser source 10 to sequentially emit light beams of various frequencies, such as the first and second frequencies described above. Control block 50 also directs polarizing device 20 to impart one of several polarizations to the beams emitted from 10, such as the first and second polarizations described above. Polarizing device 20 could consist of one or more linear or circular polarizers, with the number and type of polarizers depending upon the requirements of the specific PMD measurement method used. The light beams pass through the device being tested 30, such as a section of fiber, and are captured in polarization measuring device 40, which could be a polarimeter. Polarization measuring device 40 then measures the output polarization states of the light beams and passes this information to analysis block 60. Analysis block 60, which could be a computer, then calculates the PMD according to the algorithm used by the specific method.
This procedure for taking measurements and its corresponding apparatus have the deficiency that the measurements used to make each PMD determination are taken sequentially and are thus separated in time. Since PMD can change with changes in ambient temperature and external stresses on the fiber, the PMD of an optical device can change with time and thus will change during the time interval in which these measurements are taken. Consequently, the larger the time interval in which the measurements are taken, the larger the inaccuracy in PMD determinations made using the measurements.